A radiation measurement device is configured with a radiation detector and a measurement unit; when a radiation enters the radiation detector, the radiation detector outputs an analogue pulse with a wave height corresponding to the energy of the incident radiation. The radiation detector of a radiation measurement device installed in the vicinity of a facility such as a nuclear reactor plant or a spent fuel reprocessing plant needs to measure a radiation dose corresponding to the environmental background level of several tens nGy/h or several tens nSv/h. Accordingly, as the radiation detector, a NaI(Tl) scintillation detector or a CsI(Tl) scintillation detector is utilized; the measurement range thereof is from 10 nGy/h to 10 μGy/h or 10 nSv/h to 10 μSv/h and the detection sensitivity thereof is high.
Each of these scintillation detectors outputs an analogue pulse with a wave height in proportion to the energy of an incident radiation. The measurement unit thereof receives the analogue pulse and outputs a wave height spectrum every preset constant time, while allocating the wave heights of the analogue pulse to respective channels, the number of which is preliminarily set. Radiation energy values are allocated to the respective channels for the wave height spectra, the energy values are weighted with doses (dose equivalent quantities), and the weighted energy value is multiplied by the count of the channel so that the dose (dose equivalent quantity) for each channel is obtained. Counting is performed with regard to the channels within an energy range to be measured so that the counted dose (dose equivalent) is obtained; then, the counted dose (dose equivalent) is integrated for the measurement time so that the integral dose (dose equivalent) is obtained. Furthermore, the integral dose (dose equivalent) is divided by the measurement time, so that the dose rate (dose equivalent rate) is outputted. The wave height spectrum is measured and the dose (dose equivalent) is weighted in accordance with the wave height, i.e., the energy of a radiation, so that the energy characteristic of the output dose (dose equivalent) is compensated (for example, Patent Document 1).
In contrast, in a facility such as a nuclear reactor plant or a spent fuel reprocessing plant, as the radiation detector, a Si-semiconductor radiation detector that, as an area monitor having the measurement range of 1 μSv/h to 105 μSv/h, can cover a wide range is utilized. A Si-semiconductor radiation detector is typified, for example, a Si-PIN photodiode. A bias voltage, a reverse voltage, is applied to a Si-PIN photodiode; an electron and a hole that are produced, for example, from a γ-ray that entered a depletion layer (I layer) are collected, respectively, by a cathode electrode (N-layer) to which a positive voltage is applied and by an anode electrode (P layer) to which a negative voltage is applied. A preamplifier converts an analogue pulse current into an analogue pulse voltage; then, the radiation detector outputs the analogue pulse voltage. In a Si-semiconductor radiation detector, the counting rate sensitivity to a dose (dose equivalent) depends on the energy of an incident γ-ray; the detection efficiency is in inverse proportion to the energy and varies by approximately 1 digit within a common measurement energy range for a γ-ray, i.e., the range from 50 keV to 3000 keV. Therefore, in some radiation measurement devices, the sensitivity to low energy is lowered by a physical filter so as to match the sensitivity to high energy, so that the counting rate sensitivity to a dose (dose equivalent) is flattened (for example, Patent Document 2).